Anaerobic treatment is a known process for producing biogas and other valuable organic compounds from waste water. However, waste water often contains sulfate (SO42−) or other sulfur compounds, which, under the anaerobic conditions, are biologically converted to sulfide (mostly as HS). High concentrations of sulfide are toxic for the anaerobic biomass; they inhibit the methane formation and thus hamper the anaerobic treatment. Sulfide levels leading to 50% inhibition are 250 mg S/l in the pH range 6.4-7.2 and 90 mgS/l at pH at 7.8-8.0 (Koster et al. Water Res. 20 (12), 11561-67, 1986).
Thus, when treating waste water containing high concentrations of sulfate, it is important to control the sulfide level in the bioreactor to prevent inhibition. Waste waters containing high sulfate concentrations can be found for example in the fermentation industry (e.g. yeast and ethanol production), paper industry and starch processing industry.
In addition to inhibiting the anaerobic conversion process, sulfide resulting from sulfate and other sulfur compounds in the waste water partly ends up as hydrogen sulfide (H2S) in the product gas of the anaerobic process, i.e. the biogas, and has a negative effect on the biogas quality and on the pollution reduction efficiency. Technologies are available for downstream removing the H2S from biogas in order to make the biogas available for further processing. Some of these technologies pollute the biogas with predominantly nitrogen and some oxygen or require an excessive amount of caustic or other chemicals. In contrast, THIOPAQ® technology as described e.g. in EP 0487705, EP 0561889, and U.S. Pat. No. 6,656,249 is an environmentally friendly method for removing H2S from biogas without polluting the biogas and without excessive caustic consumption, wherein the H2S is scrubbed from the biogas and is biologically oxidised to elemental sulfur.
EP 0331806 suggests to wash H2S from the biogas and convert it by chemical or biochemical means, and to recycle part of the washed biogas to the anaerobic reactor in order to assist the mixing of the anaerobic reactor content.
WO 98/00191 discloses an anaerobic sulfur removal process, wherein the sulfide-containing effluent of the anaerobic process is circulated to a stripper column wherein the sulfide is stripped from the effluent using a desulfurised gas (biogas) originating from the anaerobic reactor. The gas is desulfurised by oxidation to elemental sulfur using iron salts. FR 2484990 discloses a similar system using an unspecified desulfurisation device.
However, the prior art does not consider the use of a column for improving the performance of the anaerobic process. The prior art processes do not provide for accommodating fluctuations of pH and toxic components in the influent of the anaerobic reactor, and do not allow high hydraulic flows because of insufficient mixing in the anaerobic reactor and hence they cannot be optimally operated.
Therefore, it is an object of the present invention to provide a process and an equipment for anaerobic treatment of waste waters containing appreciable levels of sulfate or other sulfur compounds with improved treatment efficiencies and minimised sulfur contamination levels in the ultimate gaseous and liquid effluents.